Various imaging techniques have been employed for detecting and locating cancerous tumors in body tissue. X-ray and ultrasound imaging techniques are commonly utilized in screening for breast cancer. X-ray mammography is the most effective current method for detecting early stage breast cancer. X-ray mammography, however, suffers from relatively high false positive and false negative rates, requires painful breast compression, and exposes the patient to low levels of ionizing radiation.
Microwave based imaging methods have been proposed for use in imaging of breast tissue and other body tissues as an alternative to current ultrasound and X-ray imaging techniques. Microwave imaging does not require breast compression, does not expose the patient to ionizing radiation, and can be applied at low power levels. Microwave-based imaging exploits the large contrast in dielectric properties between normal and malignant tissue. With microwave tomography, the dielectric-properties profile of an object being imaged is recovered from measurement of the transmission of microwave energy through the object. An alternative microwave imaging approach is based on radar methods that use the measured reflected signal to infer the locations of significant sources of scattering in the object being imaged. Radar methods require the focusing of the received signal in both space and time to discriminate against clutter and to obtain acceptable resolution. This may be accomplished with an antenna array and ultra-wideband microwave signals. For a discussion of this approach, see, S. C. Hagness, et al., “Two-Dimensional FDTD Analysis of a Pulsed Microwave Confocal System for Breast Cancer Detection: Fixed Focus and Antenna-Array Sensors,” IEEE Trans. Biomed. Eng., Vol. 45, December, 1998, pp. 1470-1479; S. C. Hagness, et al., “Three-Dimensional FDTD Analysis of a Pulsed Microwave Confocal System for Breast Cancer Detection: Design of an Antenna-Array Element,” IEEE Trans. Antennas and Propagation, Vol. 47, May, 1999, pp. 783-791; S. C. Hagness, et al., “Dielectric Characterization of Human Breast Tissue and Breast Cancer Detection Algorithms for Confocal Microwave Imaging,” Proc. of the 2nd World Congress on Microwave and Radio Frequency Processing, Orlando, Fla., April, 2000; and X. Li, et al., “A Confocal Microwave Imaging Algorithm for Breast Cancer Detection,” IEEE Microwave and Wireless Components Letters, Vol. 11, No. 3, March, 2001, pp. 130-132.
Alternative tumor detection and location methods have been proposed using radar methods. One method uses space-time beamforming. E. J. Bond, et al., “Microwave Imaging Via Space-Time Beamforming for Early Detection of Breast Cancer,” IEEE Trans. Antennas and Propagation, Vol. 51, No. 8, August 2003. See also U.S. published patent application 2003/0088180 A1, “Space-Time Microwave Imaging for Cancer Detection,” published May 8, 2003, the disclosure of which is incorporated herein by reference. As an alternative method, a generalized likelihood ratio test is used to detect and to locate possible tumors. See U.S. patent application Ser. No. 10/942,115, “Microwave-Based Examination using Hypothesis Testing,” filed Sep. 15, 2004, the disclosure of which is incorporated herein by reference.
In implementing these microwave breast imaging methods, the breast surface defines the imaging domain of interest. Additionally, the reflection from the breast surface dominates the initial portion of the received signal and should be removed to allow for the detection and for the location of tumors within the breast tissue. As a result, a number of microwave breast imaging algorithms rely on a knowledge of the breast surface location relative to the transmitting and receiving antennas. Unfortunately, the location of the breast surface generally is unknown a priori, is expected to vary from patient to patient, and may vary from antenna to antenna depending on the arrangement of the antenna array. Therefore, it is desirable to be able to locate the position of each antenna relative to the breast surface. It is further beneficial to use the information available from the reflected microwave signals themselves to determine the location of the breast surface.